Whether today's temperature is low enough might seem like a funny question to ask, but if you are growing fruit trees or berries, it's important to wonder if it's cold enough. Sufficient cold temperatures can mean the difference between an abundant fruit crop or little-to-no flowers or fruit.

Each year the shorter days and cooler temperatures of Fall cause many trees to produce hormones to slow or stop growth, and the plants move into dormancy for the winter. After a sufficient period of exposure to cold temperatures these hormones break down and the dormancy period ends. This change allows buds to form into flowers and foliage as the days lengthen and temperatures warm.

The time of exposure to cold is known as chill hours. A chill hour refers to any hour during which the temperature is lower than 45°F. Different plants have different requirements for the number of chill hours needed to end dormancy, called its ‘chill accumulation'. If a fruit tree or berry doesn't have sufficient chill accumulation during the season, the end of dormancy can be delayed. A late dormancy means the plants will leaf out later in the season, with fewer flowers and less or poorer quality fruit. Not only do different fruit trees have different chill requirements, but sometimes even different cultivars of the same varieties will require a different number of chill hours for abundant, healthy fruit.

Most of Northern California receives 800 to 1500 chill hours each season. In Contra Costa County the number of chill hours varies among our micro-climates. To estimate how many chill hours we've had so far, UC Davis offers a calculator at https://fruitsandnuts.ucdavis.edu/chill-calculator. This calculator provides historical data as well as for the current season from local weather stations. Choose Contra Costa and then the station closest to your location to see the totals. For example, as of this writing Concord has had 529 chill hours this season.

General chilling requirements for a variety of trees and berries in Contra Costa County can be found on the UC Cooperative Extension website at https://homeorchard.ucanr.edu/The_Big_Picture/Tree_Selection/. When you look at it, you'll see the chill accumulation requirements vary significantly by tree or berry type. For example, some varieties of pear can require 1200 to 1500 chill hours, while pomegranates only need 200 to 300 accumulated chill hours.

The number of chill hours is not the only factor that affects fruit production, although it is an important one. But knowing chill hour requirements is especially important when you are selecting a fruit or berry variety to add to your garden because that information can help you make the best choice for long term success. While there are other actions you can take to improve fruit set and quality, once your trees are in the ground you can't impact weather patterns!

Photo: R. Harper

Help Desk of the UC Master Gardeners of Contra Costa County (RDH)